Unsaturated styrenic block copolymers such as styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene (SIS) are often difficult to use in extrusion equipment as a result of thermal degradation of the copolymers during the extrusion processes. The term “unsaturated” refers to the midblock portion of the block copolymer. While SIS materials may have less difficulty than SBS materials, the SIS copolymers often lose molecular weight during extrusion, thereby leading to loss in “tension” performance in produced filaments or films.
Such block copolymers are available from numerous polymer suppliers, such as from Kraton Polymers U.S. LLC of Houston, Tex. and Septon Company of America, Pasadena, Tex. Other suppliers of such polymers include Dexco Polymers of Houston, Tex. and Dynasol of Spain. These polymers offer the potential advantages of elasticity at a relatively low cost, and at significantly lower cost than saturated styrenic block copolymers (i.e. hydrogenated midblock portions), which are currently suggested for use to impart elasticity in personal care product applications.
While such unsaturated polymers have frequently been used in adhesive-type applications, and in some instances, film applications based on solution formation, as well as for injection molding applications, such polymers have heretofore posed a manufacturing challenge in the film and filament extrusion/manufacturing setting. In particular, the unsaturated styrenic block copolymers SBS and SIS include a midblock which is unsaturated. As a result, these resin types change with processing temperature changes and lengthy dwell times in extrusion systems, such as cast or blown film systems, or filament extrusion systems. SBS tends to crosslink (increasing in viscosity and modulus) while SIS chain scissions (tending at times to lower in viscosity). SBS may even form a gel-like material during such extrusion processes. In any event, the stability of such materials over the extended period of time required to extrude film and filaments heretofore has not been predictable. Such materials may clog the die heads or the channels delivering the polymer to the die heads, resulting in a complete shut down of a manufacturing line. Since the extrusion processes for film and filament-based material may occur over a period of as long as 30-60 minutes and under temperature conditions of up to 450° F. (230° C.), the processing demands on such polymers are significant. It should be noted that while actual extrusion of polymers occurs rather quickly, the 30 to 60 minute time window describes the manufacturing process from start to finish, taking into account the polymer's travel through hoppers, melt pumps, channels, dies and also including the inevitable “down time”, where processing has to be halted for a time for some reason. Essentially, if the polymer resin has time to sit idle in the processing channels, severe manufacturing disturbances could occur.
It has been determined that even with the addition of polymer stabilizers, such as those available from Ciba Specialty Chemicals of New Jersey, under the designations IRGAFOX 168, IRGANOX 1010, 1076, and ULTRANOX 626, and SUMILIZER GS from Sumitomo, little improvement can be provided to address these processing issues. While such stabilizer packages may extend the stability extrusion time a few minutes, they do so at much additional processing cost, thereby adding significant costs to the end product. The narrow processing temperature window required for these polymers has therefore limited their practical usage in consumer product applications.
As can be seen in FIGS. 1 and 2, which illustrate the effects of various stabilizers on such polymers during extrusion, including a comparison to easily processed saturated stryrenic block copolymers without stabilizers (KRATON G 1730), unsaturated styrenic block copolymers demonstrate processing challenges, even with the addition of such stabilizers. Such materials were extruded as films, and then evaluated in a time sweep test method as described below. As can be seen in FIG. 1, a saturated styrenic block copolymer 2, retains its viscosity at a fairly constant level in an extrusion system over a lengthy period of time, even without use of a stabilizer. However, the addition of various stabilizers 3, 4, 5, and 6 to KRATON D1402 SBS (#4 which was 1250 parts per million (ppm) of IRGANOX 1010 along with 2500 ppm IRGAFOX 168, #6 which was mineral oil (between 5-10 percent), #3 which was 750 ppm of IRGANOX 1010 along with 1500 ppm of IRGAFOX 168, #5 which was 1000 ppm SUMILIZER GS) fail to control viscosity levels over the same time period.
It can be seen in FIG. 2, that some stabilizers 7, 8, 9, 10 do have more of a leveling impact on KRATON DRP 6430 SIS materials over time, as such materials retain a more predictable viscosity level, and one that is consistent with a saturated styrenic block copolymer (KRATON G1730) 2. In FIG. 2, the stabilizers included 1250 ppm IRGANOX 1010 and 2500 ppm of IRGAFOX 168 (8), 1000 ppm of IRGANOX 1010 and 2000 ppm of IRGAFOX 168 (7), no stabilizer (9), and 750 ppm IRGANOX 1010 and 1500 ppm of IRGAFOX 168 (10). However, it has been found that even with such stabilizers, SIS materials do not always demonstrate sufficient elastic properties by themselves, that are desirable in personal care product applications. For instance, SIS materials tend to lose molecular weight/tension as they are being processed. Therefore more of the material has to be utilized to maintain tensions at acceptable levels in final product. Also, as can be seen, as a result of loss in molecular weight, the viscosity of such materials may lower over time.
As such polymers either cross-link or chain scission, the viscosity of the polymers during the extrusion melt fluctuates with only minor temperature variation. It is therefore difficult to maintain a relatively constant viscosity of such polymers in an extrusion process so as to produce a film or filament having uniform properties throughout. For instance, even if the polymer could successfully pass through the manufacturing system, slight variations in film production (casting) temperatures could cause film edges to tear, film hole formation and general structural unevenness. Therefore, manufacturing difficulties have effectively made use of such polymers for various consumer product applications impractical.
There is therefore a need for acceptable polymer compositions that utilize unsaturated block copolymers. There is also a need for methods of processing such polymers that avoid unwanted chemical reactions during processing, making use of such polymers more practical. There is also a need for methods for processing such polymers into film and filament/fiber applications, in which relatively uniform viscosity is maintained over a broader range of processing temperatures and longer dwell times (that is the time it takes for the polymer to pass through the entire extrusion system, even with manufacturing delays). There is likewise a need for films and filaments made by such compositions rather than films and filaments made from more costly saturated styrenic block copolymers. There is also need for film and filament/fiber materials made from such compositions that offer acceptable elastic performance for use in a variety of personal care product applications. Finally, there is a need for unsaturated block copolymer compositions that are capable of being combined with other lower cost polymers, such that overall manufacturing costs may be reduced, without compromising product quality or performance. It is to such needs that the present invention is directed.